3. The secant modulus of elasticity of acrylic plastic after 48 hours 

 of sustained tensile loading in the 75 to 77°F range and at a 6,000-psi stress 

 level is 90,000 psi; at a 4,000-psi stress level it is 200,000 psi; at a 3,000-psi 

 stress level it is 300,000 psi; and at a 2,000-psi level it is 400,000 psi. 



4. The strength and modulus of elasticity increase as the temperature 

 of acrylic plastic is decreased from room temperature (75 to 77°F range). 

 The increase is about 10% for the 30 to 35°F range. 



5. There is no change in the impact strength of unnotched acrylic 

 plastic bars with the decrease in temperature from the 75 to 77°F range to 

 30 to 35°F range. 



6. The secant modulus of elasticity of acrylic plastic under sustained 

 compressive loading in the 75 to 77°F range is 10 to 20% higher than for 

 sustained tensile loading. 



7. The compressive short-term strength of acrylic plastic is approxi- 

 mately 1 5,000 psi, while the tensile strength is 9,000 psi. 



The data generated in the NCEL hydrospace window research program 

 was less voluminous than in the aerospace research program but it had the 

 advantage of being more directly applicable. In this research conical, flat disc, 

 and spherical acrylic plastic windows were subjected primarily to compressive 

 stresses, which either caused the windows to fail because of excessive stress 

 magnitude or plastic instability. The results from the hydrospace window 

 research program applicable to the capsule design were: 



1 . Time-dependent ultimate failure of acrylic plastic structural 

 components by viscoelastic or viscoplastic instability caused by sustained 

 compressive loading occurs in less than 24 hours only when the loading level 

 is generally 3*70% of the short-term compressive strength. When the loading 

 is<25%, the ultimate failure will definitely not occur in 10,000 hours. 



2. Cyclic compressive loading may cause ultimate fatigue failure of 

 acrylic plastic structural components in less than 100 cycles only when the 

 loading is >50% of short-term compressive strength. When the loading is 

 <25%, the ultimate failure will definitely not occur in 1 ,000 cycles. 



From the rather general and only indirectly applicable information 

 available at the time of design of the NEMO acrylic hull, two general con- 

 clusions could be made: 



1 . If the maximum operational depth of the hull was chosen to be 

 at 20 to 35% of its implosion depth under short-term loading conditions the 

 hull would probably be safe for long-term cyclic and sustained pressure ser- 

 vice at the maximum operational depth so long as the fatigue life was not to 

 exceed 1 ,000 dives of 8-hour, or 1 00 dives of 1 00-hour duration. 



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